This invention relates to materials science and, more particularly, to chemical vapor deposition (CVD) and chemical infiltration (CVI). This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
This invention is a method for making compositions by means of CVI and an inverted temperature gradient, where microwave energy is the source of heat for the process. Currently used commercial CVI processes involve conventional heating of a substrate, or preform, in an atmosphere containing gases which react to form a solid ceramic material. The reactant gases must be transported into pores, or open spaces, within a substrate and gaseous products of the reaction must be transported outward from the interior of the preform. In conventional heating processes, the exterior surfaces of a substrate are necessarily at the same temperatures or at higher temperatures than locations within the substrate. Thus, reaction of gases to produce the solid ceramic material tends to take place in pores near the surface of the substrate and thereby close off passageways for gas to diffuse into locations near the center of the substrate. Weeks and months are usually required to produce an article of any size and often the degree of densification of the article is unsatisfactory. Also, it is often necessary to interrupt a CVI process to subject an article to diamond machining to reopen passageways into the center region of the article in order that deposition in and densification of the center region may continue. A method for CVI which is currently being developed at Oak Ridge National Laboratory is referred to as the thermal gradient forced flow method, where one end of an article is heated and reactant gases are forced through the article from its cold end. This method reduces processing time to days but is restricted to simple shapes.